Origins of Angiosperms
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Origins of Angiosperms
Spring 2014
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Outline• Origin of the angiosperms • Characters of angiosperms• Brief history of angiosperm
classification• Major groups of angiosperms• ANITA grade
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Origin of the Angiosperms
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ConifersGinkgoCycads Gnetophytes Angiosperms
Divergence estimated at ca. 325 mybp
Progymnosperms(“seed ferns”)
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Geologic Time
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Timing of Angiosperm Divergences
• The timing of the origin of the angiosperms still is uncertain, but most would agree on a Triassic or Jurassic initial divergence, although there is no unequivocal fossil evidence
• A demonstrable “burst” of phylogenetic radiation is found in the fossil record beginning in the mid- to late Cretaceous, 140 - 100 mybp
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Figure 7.1
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Origin of the Angiosperms
• pollen grains from ca. 140 mya (early Cretaceous) but already major radiation!
• earliest flowers 130 mya
• likely no extant group of seed plants is very closely related to the angiosperms!
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Origin of Angiosperms• Cycad-like plants: Bennettitales?• large, flowerlike strobili:• pollen-producing organs surrounding an axis
bearing naked ovules/seeds
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Origin of Angiosperms
Modification of a “seed fern” such as Caytonia?
Caytoniafossil: ovule
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Origin of Angiosperms
• Archaefructus
• ca. 130 mya• ancestral flowering plant or extinct off-shoot of an extinct lineage?• aquatic plant (dissected leaves)• elongate reproductive axes:
-paired stamens below-several-seeded carpels above
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Characters of Angiosperms
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Fig. 6.1
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What makes a plant an angiosperm?• Flower (usually with perianth)• Carpels with a stigmatic surface for pollen tube
germination; ovules enclosed within carpels; fruit• Ovules with two integuments• Reduced female gametophyte, usually 8 nuclei in 7 cells –
no archegonium• Double fertilization with the production of 3N endosperm• Stamens with two pairs of lateral pollen sacs
(microsporangia); pollen (male gametophyte) 3-nucleate• Xylem – most with vessels (evolved within angiosperms)• Phloem – sieve tube members with 1 or more companion
cells derived from the same mother cell
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FlowerFigure 6.2
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Spiral undifferentiatedperianth parts = tepals(plesiomorphic)
Magnolia
Magnolia
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Differentiated sepals andpetals (each in whorls)(apomorphic)
Abutilon
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connectivemicrosporangium
filament
Laminar stamensin basal angiosperms(plesiomorphic)
paired pollen sacs
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Early carpel with stigmatic crest(plesiomorphic)…
…to the derived carpelwith a style and an apical stigma (apomorphic).
Figure 6.9 from the text
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Female gametophyte in angiosperms
ovule
-no waiting time as in gymnosperms!-note 2 integuments (bitegmic; some angiosperm lineages have lost one integument) -gymnosperms have only 1 integument (unitegmic)
mature ovule
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And it’s off to the races!
Indirect pollination(due to presenceof carpels)
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-no waiting time as in gymnosperms!
Seed development in angiosperms
seeddouble fertilization
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Avocado (Persea, Lauraceae)
seed
endocarpmesocarp
exocarppericarpFlower
fruit
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• are the water (solute) conducting cells of the
xylem in most angiosperms• ends of cells have openings (perforation plate), cells shorter and wider• more efficient, faster rate of flow but more susceptible to air bubbles (embolisms) than tracheids are• may have arisen independently in two or
more angiosperm lineages but may have had a single origin
Vessels in Angiosperms
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Origin of vessels from tracheids
Figure 6.16B from the text
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Angiosperm phloem
• stm = specialized sugar-conducting cells of the phloem of angiosperms; lack a nucleus at functional maturity
• cc = parenchyma cells associated with stm
-function to load/unload sugars into stm cavity-derived from the same mothercell as its stm
• sieve tube members + companion cells
stm
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Brief history of angiosperm classification
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Alternative ways of thinking about early angiosperm characters…
• “Old” School (German) - Engler - “Simple is primitive” (Few floral parts)- Ancestors are conifers- Pollination by wind- Modern relicts = “Amentiferae” (catkins)
• “New” School (American) – Bessey- “Flowers with many parts are primitive”- Ancestors are Cycad-like plants- Pollination by primitive insects- Modern relicts = Magnolias and allies
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Heinrich Gustav Adolph Engler(1844-1930)
- German Botanist at Berlin Botanical Garden
-Was the primary European in interpreting the grouping of major angiosperm assemblages-“Few simple flower parts primitive”-Small, unisexual flowers primitive
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Englerian ‘Primitive Taxa’
“Amentiferae”
Quercus sp.
Juglans sp.
Betula sp.
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Charles Edwin Bessey(1845-1915)
-Botanist at Iowa State University from 1869-1884 (left in 1884 to teach in Nebraska)-Was a “major player” in interpreting and understanding angiosperm evolution-“Many flower parts primitive”
Bessey HallIowa State University
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Bessey’s “Cactus”(1915)
Placed plant groupswith many floral parts in a basalposition as the‘ancestral’ forms.
Outlined ‘dicta’ for the construction ofphylogenies using theevolutionary trends incharacter changes.
Polypetalous flowers,insect pollination, cycad-like ancestors
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Besseyan ‘Primitive Taxa’
Nymphaeaceae
Magnoliaceae
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Figure 6.1 from the text
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Major Groups of Angiosperms• Basal Angiosperms (ANITA grade)
- Amborellaceae- Nympheaceae- Illiciaceae
• Magnoliid Complex- Magnoliales- Piperales- Winterales
• MONOCOTS• EUDICOTS (tricolpates)
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Major Groups of Eudicots• Basal Eudicots
- Ranunculales and allied families
• Caryophyllales & Saxifragales• Rosid Clade
- Fabids- Malvids
• Asterid Clade- Basal Asterids - Lamiids- Campanulids
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ANITA grade
• Amborella (Amborellales)• Nymphaea (Nymphaeales)• Illicium (Austrobaileyales)• Trimenia (Austrobaileyales)• Austrobaileya (Austrobaileyales)
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ca. 125 mybp
Fig. 6.1
Grade = a paraphyletic (orPolyphyletic) group whosemembers share a similar level of morphological orphysiological complexity.
ANITA Gradeor “basal”angiosperms
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Major Angiosperm Clades
Amborellaceae
Nymphaeales
Austrobaileyales
MAGNOLIID COMPLEX
MONOCOTS
EUDICOTS [TRICOLPATES]
ANITA grade (basal
groups)
Soltis et al. 2000, APG II 2002, Judd et al. 2002
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Basal Angiosperms: Amborellaceae
• New Caledonia• Understory shrub; plants dioecious• 1 species (monotypic): Amborella trichopoda• Leaves simple, evergreen• Flowers small, unisexual: ♀ apocarpous, with
stigmatic crests; ♂ with laminar stamens• Significant features: Most basal of all flowering
plants; no vessels in wood• Special uses: (none)
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Basal Angiosperms:Amborellaceae (Amborella Family)
Amborella trichopoda
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As we venture through the various major groups of
angiosperms…• Identify the plesiomorphic characteristics
associated with particular groups and note their apomorphies (if any) as well.
• Try to associate “syndromes” of characteristics with each group (make note of special characters occurring together).
• One good way to study is to write keys to the groups we cover in any given unit.
• Names of groups are important! Learn to spell and say them!
• Ask questions!!